1. Field of the Invention
The present invention provides a method to prepare fluorocarbon surfaces for bonding, as for example in electroless and electrolytic plating. This is particularly useful in the preparation of circuit boards. The surface of a fluorocarbon material does not normally permit bonding and wetting. The surface of fluorocarbon materials e.g. fluorocarbon resins is both hydrophobic and oleophobic. Because of this, only liquids with low surface tensions are capable of wetting fluorocarbon surfaces.
2. Description of the Art
Fluorinated polymers such as polyvinyl fluoride (PVF) and polytetrafluoroethylene (PTFE) are characterized by extreme interness, high thermal stability, hydrophobicity, and a low coefficient of friction. Because of these properties, these and other fluorocarbon polymers resist adhesion to almost any material. The properties of fluorocarbon materials are highly desirable for many industrial and consumer applications. However, it may be necessary to modify certain characteristics of fluorocarbon materials to enable the fluorocarbon polymers to be bonded to different materials, or surface coated, such as, for example, in the manufacture of circuit boards as hereinabove noted.
The following are representative of the methods used presently to bond and wet the surface of fluorocarbon polymers, which methods are either very tedious or unfriendly to the environment or very uneconomical and impractical.
1) Vapor Blasting: This method increases the roughness of the surface, which lowers the contact angle. This method however gives very unsatisfactory peel strength, when other materials are adhesively bounded to a fluorocarbon surface.
2) Corona Blasting: This method is effective for use with certain fluorocarbon polymers, such as fluorinated ethylene-propylene copolymers (FEP). In this method, high frequency; high voltage and alternating current is used to produce a corona discharge. Some modifications may be made to make this process useable with other fluorocarbons such as PTFE.
3) Molten Alloy Treatment: Treatment of the fluorocarbon in a molten alloy of tin and lead (63/37%), respectively, with sodium, is carried out at a temperature below a eutectic point of the alloy.
4) Sodium Anhydride Treatment: In this method, the fluorocarbon polymer is treated with sodium anhydride and then subjected to a helium atmosphere at 350xc2x0 C.
5) Sodium Ammonia Treatment:
The fluorocarbon material, according to this treatment, is immersed in anhydrous ammonia and an alkali metal like sodium. The process is very unattractive because of ammonia fumes and the hazardous nature of anhydrous ammonia and sodium.
Examples of the above methods of treating fluorocarbon surface, as well as other methods for increasing the wettability of fluorocarbon surfaces, are found in the following patents:
U.S. Pat. No. 5,627,079 to Gardella, Jr., et al. discloses a method of increasing the wettability of a fluorocarbon film by treating such film with a gas/vapor mixture of H2 and one of H2O, CH3OH or H2CO under vacuum and in the presence of a radio frequency glow discharge.
British Patent 1,043,233 discloses etching fluorocarbon with polyarylsodium followed by depositing electroless copper on the etched fluorocarbon.
U.S. Pat. No. 2,789,063 discloses etching a fluorocarbon surface with a fluid metal reactant, e.g. lithium vapor, liquid sodium, a liquid alloy of Pbxe2x80x94Snxe2x80x94Na, NaH or CaH2 
British Patent 890,466 discloses treating a perfluorocarbon resin with a corona discharge followed by applying an adhesive and laminating with copper.
U.S. Pat. No. 5,580,616 to Niino discloses treating a fluorocarbon polymer surface with UV in the presence of hydrazine to make said surface hydrophilic. (The hydrophilic surface can be treated chemically to deposit a metal film thereon or made lipophilic by treating with an acid anhydride).
Fluorocarbon materials are used in many applications, e.g. in the manufacturing of printed circuits boards, the manufacture of co-axial cables having polytetrafluorethylene as the dielectric, etc. Several methods are used to provide conducting finishes and necessary surface preparations to these fluorocarbon materials. These methods include the metallization or surface treatments described above. However, the above methods of treating fluorocarbon materials have disadvantages which are overcome by the method of the present invention.
In particular the disadvantages of the prior art methods for activating fluorocarbon surfaces include:
High vapor pressure builds due to the volatile solvent used in most of the chemicals of the prior art;
Most of the chemicals of the prior art have a low flash point;
The chemicals of the prior art require refrigerated storage in most of the cases;
Many of the chemicals are very sensitive to moisture and air contamination; and
The chemicals of the prior art have very limited shelf life, for example a maximum of about 2 hours, particularly when the chemicals are exposed to air, mixed with water or contaminated with moisture.
The present invention provides a method of activating a fluorocarbon surface to render such surface active to metal coating and/or organic adhesives or inks, which comprises contacting said surface with a mixture comprising a cyclic amide, a quaternary ammonium compound, a cationic or nonionic surfactant, a glycol ether and an organic acid.
The chemicals used in the method of the present invention are a mixture of a cyclic amide, a quaternary ammonium compound, a cationic or non-ionic surfactant, a glycol ether and an organic acid.